The following discussion of the prior art is intended to place the invention in an appropriate technical context and to allow its benefits to be fully appreciated. Any statements about the prior art should not, however, be considered as admissions that such prior art is widely known or forms part of common general knowledge in the field.
Conventional flotation devices typically include a tank for receiving and containing slurry from a grinding mill, cyclone separator, or the like. An agitator, comprising a rotor housed within a stator, is normally disposed within the tank, and activated via a motor and drive shaft to agitate the slurry. An aeration system is also provided to direct air under pressure into the agitator through a central conduit formed within the drive shaft. Suitable reagents are also added, which coat the surfaces of the mineral particles within the slurry to make the particles hydrophobic and thereby to preferentially promote bubble to particle attachment. As bubbles dispersed by the rotor rise toward the surface of the tank, they carry with them floatable valuable mineral particles, which form a mineral enriched surface froth. The froth then migrates over a lip and into a launder whereby the valuable mineral particles suspended in the froth are recovered from the tank as a mineral concentrate. The gangue particles remaining suspended in the slurry, along with those mineral particles that were not removed by flotation, are continuously discharged from the tank through a bottom outlet. The bottom outlet often incorporates a dart or pinch valve, which is opened to allow the remaining slurry to progress under gravity feed to downstream treatment processes. It is normal practice to control the pulp level in each device using a PID controller, a level indicating probe and a control valve in the form of a dart, pinch or other suitable type of valve.
The slurry that is transferred through the bottom outlet includes both relatively coarse or dense particles as well as a large number of relatively fine particles, including gangue slimes such as clay minerals, not removed by flotation. The slimes consist of very fine particles and accordingly have a total surface area much greater than that of the coarse particles. Accordingly, when a flotation reagent is added to the outflow from the tank, the majority tends to be absorbed by the slimes, which are not floatable, making the flotation process non-selective. Consequently, most of the coarser valuable particles do not receive sufficient flotation reagent to make them hydrophobic, even given extended conditioning times.
The flotation process can be made more efficient where coarse and fine particles are treated separately and in the past, devices such as hydrocyclones and hydrosizers have been used to separate a flotation feed stream into two discrete streams for separate processing. However, the capital cost of this equipment is high, making the prior art methods uneconomical for all but the most valuable ore bodies.
It is an object of the present invention to overcome or substantially ameliorate one or more disadvantages of the prior art, or at least to provide a useful alternative.